Magnetic core



Get. 25, 1949. s. Q. so ERvaLLE MAGNETIC CORE 3 Sheets-Sheet 1 Filed Oct18, 1947 Irwverwtor: Gareth G. Somevvlile,

by Has Attorn ey 1949- I G. G. SOMERVILLE 2,435,220

MAGNETIC CORE Filed Oct; 18, 1947 3 Sheets-Sheet 2 Imverfitor: Garet. G,Somervil ie,

is y.

Oct. 25, 1949.

Filed 001:. 18. 1947 FLUX 9o DENSITY o.

(- KILGL/NES 70 PER sqmncl G. G. SOMERVRLLE MAGNETIC CORE 3 Sheets-Sheet3 XC/TlN6 CURRENT Fig. 52.

8O KILOLINES PER SQUARE INCH EXCITING CURRENT JOINT OPENING INC/IE3 Fig.

3 I00 KILOL/NES :4 PER SQUARE 1mm 8 ace/551.5

8 i g S FLUX DENSITY (K/LOL/NES PER SQUARE we) Invent/0r:

Gareth G Scmerville His Attorney.

Patented Oct. 25, 1949 MAGNETIC CORE Gareth G. Somerville, Pittsiield,

to General Electric Company,

New York Masa, alsignor a corporation of Application October 18, 1947,Serial No. 780,705 7 Claims. (Cl. 175-356) This invention relates tomagnetic cores and more particularly to improvements in lap joints forlaminated magnetic cores.

It is usually necessary to have one or more joints in closed laminatedmagnetic cores. In the case of cores which are made by stacking straightflat punchings against each other, joints are necessary at the cornersof the core. In the case of bent lamination cores, of either the woundor precut-preformed variety, which are usually characterized by one ormore straight winding legs and rounded corners, at least one core jointis desirable in order that standard preformed coils or windings can befitted onto the wind ing leg. By the term. core joint, it is meant thatall the individual lamination joints making up the core joint lie closeto the same transverse plane through the core, as distinguished from ajointless core in which the joints in the individual lamination layersare distributed widely throughthe length and breadth of the core. A corejoint is ordinarily characterized by being openable and closable and ithas a relatively short lengthwise extension compared to the length ofthe core's magnetic circuit, whereas cores having the individuallaminations provided with widely separated joints are not openable orseparable in any practical sense.

Core Joints may be classified as butt joints and lap joints. In buttjoints, the ends or sides or both of the laminations forming the jointall lie in the same surface, which is usually a plane. In lap joints,the individual joints in adjacent lamination layers are overlapped,usually in a regular pattern. Each lamination layer isone or morelaminations thick. The overlap pattern may either be recurring (i. e.repetitive or cyclic) or nonrecurring. Heretofore, the recurring patternhas been characterized by what may be called "reversibility i. e. for.every overlap or series of overlaps in one direction there is an equaland opposite overlap or series of overlaps in the opposite direction insuccessive lamination.

layers. An example of this is the standared socalled twoway overlap corejoint in which all the joints in the even numbered lamination layers arein one plane which is offset a given distance from another parallelplane in which lie all the joints in the odd numbered lamination layers.

Such reversibly recurring lap pattern core joints, while characterizedby good mechanical strength and. relatively short length, have ratherhigh reluctance unless quite carefully made and very closely'fittedtogether. p On the other hand, the nonrecurring lappat tern core jointhas poor mechanical strength, and in cores having a relatively largenumber of lamination layers, such a joint extends along such a largepart of the core length that it is impractical and there is not adequatespace for it on the core. This is because the overlap between joints inadjacent lamination layers must ordinarily be of the order ofthree-quarters of an inch, and in power transformers having hundreds oflamination layers, the total length of I? rzonrecurring pattern lapjoint will be several In accordance with this invention, there isprovided what may be called an irreversibly asymmetrically recurring lappattern core joint. Such a joint is further characterized by having athree, four, or more way lap. It has remarkably low reluctance even atrelatively wide separation or opening and it has good mechanicalstrength. The very much improved magnetic properties of the irreversiblyrecurring pattern lap joint make it practical greatly to increase thenumber of joints in a given magnetic circuit so that hithertoimpractical forms of cores are now practical. Other advantages of theimproved core joint are that it is relatively easy to put together andit is relatively quiet in operation.

An object of the invention is to provide a new and improved magneticcore.

Another object of the invention is to provide a new and improved jointfor a laminated magnetic core.

An added object of the invention is to reduce the reluctance of lapjoints of magnetic cores.

A further object of the invention is to provide a core joint whosereluctance is relatively independent of its degree of opening withinwide limits.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawings, and its scope willbe pointed out in the appended claims.

In the drawings, Fig. 1 is a perspective view. partly broken away, of aprecut-preformed Y yoke three-phase core embodying the invention; Fig. 2is a detailed view showing in enlarged form certain of the joints of thecore shown in Fig. 1; Fig. 3 is an enlarged detail view showing thejoint construction at the center of the yoke part of the core shown inFig. 1; Fig. 4 shows amodified four-way lap joint; Fig 5 shows the wrongway to assemble the laminations. so as to provide a reversibly orsymmetrically recurringv pattern lap joint; Fig. dis a front view of asuitable jig for forming the bent yoke cap portions of the core shown inFig. 1; Fig. 7 is a side view of the structure shown in fig. 6; Fig 8shows, partly in section, how the jig. of Fig. 6 forms simultaneouslytwo of the yoke cap members; Fig. 9 is a sectional view showing how thestraight leg laminations may be perforated and assembled so as toprovide the three-way lap joint shown in Figs. 1 and 2; Fig. 10 is a setof magnetization curves for comparing the magnetic properties of myimproved joint with the magnetic properties of the core steel itself andwith a standard three-quarter inch two-way lap joint core; Fig. 11 is aset of curves illustrating the relationship between exciting current andjoint opening at a flux density of 100 kilolines per square inch ior myimproved joint and a standard two way lap joint; Fig. 12 is similar toFig. 11 except that the flux density is 80 kilolines per square inch andFig. 13 is a set of curves which compares the noise made by similarcores having different joints.

Referring now to the drawings, and more particularly to Fig. 1, the coremay consist of three identical straight winding legs I whosecorresponding ends are joined together by Y-shaped yokes which areindicated generally by 2. As shown most clearly by the section of thecutaway portion of the left-hand winding leg, the core is of so-calledcruciform cross section and it consists of steps of different widthlaminations. The ends of the legs I are connected to the yokes 2 bynovel joints 3. The yokes 2 in turn may each consist of identical curvedcore members 4 and central connector or insert members 5.

The joints between the members 4 and 5 are also of the same pattern orconfiguration as the joints 3 between the legs I and the curved members4. These joints are indicated at 6.

In Fig. 2, some of the'laminations of a leg I and of two curved members4 are shown edgewise and enlarged. The upper joint 3 is shown entirelyopen and the lower joint 3 is shown tightly closed. It will also be seenthat each lamination layer consists of two lamination pieces. Inpractice, it is usual to have several laminations in each laminationlayer so as to facilitate handling of the parts as the layers becomemore rigid and easier to handle when they are built up of two, three, orfour lamination members.

Considering the joints 3 which are shown in Fig. 2, it will be seen thatthere is a joint 1 in the outer layer 8 which is ofiset in a givendirection from a joint 9 in the second layer H). In the third layer ii,there is a joint I! which is offset from the joint 9 in the samedirection and by the same amount as was the case with respect to thejoints 1 andQ. In the next layer l3, there is a joint i4 which insteadof being overlapped by the same amount as heretofore is overlapped allthe way back to the same transverse location or plane as the joint 1. Inthe next layer l5, there is a joint l6 which corresponds with the joint9 in layer l0, and'in the last layer I1, there is a joint l8 whichcorresponds in lengthwise location with the joint II. The joints 1, 9,and I! thus form a pattern which pattern is repeated in the joints l4,l6, and it, but this repetition or recurrence is asymmetrical and notreversible because the step or overlap between adjacent joints i2 and i4is the full width of the core joint instead of being merely the amountof overlap between the adjacent joints in the pattern 1-9--l2 or|4-I6-l8. This will be explained more fully in connection with Fig. 5 inwhich the lamination layers are improperly assembled to form asymmetrically reversible pattern lap joint.

One of the things to notice about the Joints 3 in Fig. 2 is thatalthough there are six lamination layers shown, the ends of theselamination layers do not all come together at the same time when thejoints close. This is clearly shown by the open upper joint in Fig. 2 inwhich the ends of the lamination layers of the curved member 4 whichform the layer joints 1 and i4 have a relatively wide space which infact is equal to twice their width or that of four lamination layers, inwhich to adjust themselves transversely as the core joint closes. Thisis in marked contrast with an ordinary two-way reversible lap patternjoint in which all of the long ends of the lamination layers must comeinto contact with each other at the same time when the joint is closed.

Another and very important feature of the joint can be seen most clearlyin connection with the closed lower joint 3 shown in Fig. 2, that is,that only one-third of the cross section of the core consists of layerbutt joints or gaps in any plane and the other two-thirds of the core inthat plane is continuous magnetic material. For example, consider theplane containing the layer joints or gaps I and I4. Those joints or gapsare in two of the six lamination layers so that in that plane the otherfour lamination layers are uninterruptedly available for carrying thecore flux. The same thing is also true for the plane containing thejoints or gaps 9 and I6 and for the plane containing the joints or gapsl2 and I8. Thus, the core is magnetically symmetrical so far as thesethree planes are concerned, and the magnetic properties or the core arethe same in each plane.

The specific reluctance of each 01 the layer joints or gaps is, ofcourse, higher than the specific reluctance of the iron or steel itself,so that what is believed to happen is that at each layer joint or gapthe flux tends to crowd into the adjacent layer or layers, thusincreasing their flux density. When it is realized that modern electricinduction apparatus operates with its magnetic cores very little belowthe knee of the saturation curve, it will be seen that not much crowdingcan take place without raising the flux density in the crowded parts ofthe core to above the knee of the saturation curve, thus producing localsaturation with the result that there is a very marked increase inreluctance and hence in exciting current required to produce the totalflux, the value of which is fixed by the voltage of the apparatus.

Fig. 3 illustrates the details oi the joints i between the center insertmembers 5 and the curved yoke member 4. The joints 8 between thesemembers are essentially the same as the joints 3. The lamination pieceswhich make up the leg I and the curved yoke members 4 are preferablymade of material such as a high reduction cold rolled silicon stripsteel which has a highly favorable magnetic direction which coincideswith the direction of rolling and which coincides with the direction ofnormal flux travel in the lamination pieces. As the flux does not alwaystravel in the same direction in the center insert pieces 5, there is notmuch, if any advantage in using the same kind of material for theseinserts. However, if directional material is used it is desirable torotate successive pieces 5 120, so that for each oi. the differentdirections of flux travel, some of the pieces will have their mostfavorable magnetic direction in line with the flux.

The three-phase Y yoke core shown in Fig. 1 is. a species of the corewhich forms part of the subject matter of an application Serial No.722,843, filed January 18, 1947, now Patent No. 2,456,461, issuedDecember 14, 1948, in the name of Cecil G. Dunn, and assigned to thepresent assignee. Also the generic idea of a core having lap jointsbetween straight stacked winding legs and fiatwise curved yoke membersis described and claimed in application Serial No. 645,650, filedFebruary 2, 1946, in the name of Ivanhoe H. Solater, and assigned to thepresent assignee.

Fig. 4 illustrates a modification of .the joint illustrated in theprevious figures, and it may be described as a four-way lap joint incontradistinction to the three-way lap joint shown in the previousfigures. In the four-way lap joint, there are four spaced planes throughthe core, each of which contains a number of lamination layer joints orgaps, the number being one-fourth of the total number of laminationlayers. In other words, in each plane through the core which containslayer joints or gaps, there is threequarters of the totalcross-sectional core area of metal which is available for carrying thetotal flux. The four-way lap joint has still better magnetic propertiesthan the three-way joint. However, it has been found that for eachincrease in the number of steps the corresponding improvement inmagnetic properties gets less and less so that the diminishing returnswill ordinarily make it uneconomical to increase the number of steps orways substantially above four.

In Fig. 5, there is illustrated a reversibly recurring three-step orthree-way lap joint pattern. Thus, individual layer joints or gaps I8,l9, and

20 progress as do joints I, 9, and I2 in Fig. 2.

However, the joint 21 in the next layer overlaps only by the amount ofoverlap between adjacent joints l8, l9, and 20, so that it is in thesame plane with joint l9. Similarly, the next joint 22 is in the sameplane with joint 18. Thus the pattern of joints 20, 2|, and 22 is justthe reverse of the pattern of joints 3, l9, and 20, and these patternsare symmetrically reversibly repeated throughout the thickness of thecore. The result is a very uneven distribution of the number of layerjoints or gaps in the three different transverse planes through the corewhich contains these joints. Thus, in the plane containing the joint orgap 20 there are only two joints or gaps, although there are ninelayers, whereas in the plane containing the joint I! there are fourjoints, while in the plane containing the joint l8 there are threejoints. Consequently, the average flux density will be quite differentin the iron in these three different planes, it being by far the highestin the middle plane containing the joint or gap l9, becausesubstantially half the area is layer joints and substantially only theother half is magnetic material which is available for carrying thetotal flux.

Fig. 6 illustrates one way of forming the curved yoke members 4, two ata time. As shown, the individual lamination pieces, which preferablyhave been automatically precut to the proper length, are stacked in theproper order in a jig which by means of side arms 23 and a clamping bar24 are held firmly in place with one joint assembled at the center. Fig.'7 shows a side view of the jig with the cruciform cross section coreparts clamped in place. The entire jig is carried by a verticallymovable rod 25 and a pair of fixed forming rollers 26 are mounted abovethe assem- 6 bly of laminations and are laterally spaced from the sidesurfaces 21 and 28 of the jig by the thickness of the assembled stack oflaminations. Consequently, as the rod is moved upward, the ends of theclamped laminations are bent downwardly into the position shown in Fig.8. A suitable clamp 29 may then be applied to hold the bent laminationsin this position, after which the assembly canbe removed from the jigand annealed in the, usual way. By properly predetermining the lengthsof the individual lamination layers, their ends will all form thedesired patterns as shown in Fig. 8.

The core legs I may be assembled readily by means of a pair of properlyspaced pins 30 in combination with suitably spaced perforations in thelaminations. Layer 8, which is the same as every fourth layer, such asthe layers 8' and 8", may be provided with a round hole near one end anda slot near the other end, the hole fitting over the upper pin 30 andthe slot fitting over the lower pin 30. The layers ll, II, and H are allalso the same as each other and, furthermore, they may be exactly thesame as the layers 8, 8' and 8" except that they are reversed lengthwiseso that their round holes fit over the lower pin 30 and their slots fitover the upper pin 30. The intermediate layers l0, l0 and I0" are alsoall alike but they are different from the other layers in that theirperforations and particularly the location of the holes to fit snuglyover one pin are positioned so as to bring the ends of theseintermediate layers l0, l0 and I0" into the proper relation to the otherlayers.

In Fig. 10, the dashed curve 3! is the magnetization curve of themagnetic material from which the core shown in Fig. 1 is made. That isto say, it represents an ideal jointless core made of the same magneticmaterial. Curve 32 is a curve of the same size core having two standardthree-quarter inch two-way lap joints. As will be seen, the excitingcurrent increases very rapidly when the flux density exceeds kilolinesper square inch. Curve 33 is for a similar core having two three-way lapjoints of the type illustrated in Fig. 2, for example. It 'will be seenthat in this case, the flux density can be carried up to kilolines persquare inch before manifestations of saturation appear and the excitingcurrent begins to increase rapidly. Curve 34 is for a similar core buthaving two fourway lap joints of the type-shown in Fig. 4. Here the fluxdensity can be carried up to kilolines before saturation beginsPresumably increasing the number of ways or laps will produce a familyof curves in between curves 3! and 34.

The data for curves 32, 33, and 34 was obtained from rectangular coreshaving two winding legs and two curved yokes, the thickness of the corebeing three inches, its width being four inches, and the dimensions ofthe central window being 6 inches by thirty-two inches. They containedapproximately 267 /2 pounds of 8X10 steel. The joints had an opening of.050 inch. By that it is meant that there was that much gap or openingor separation between the adjacent ends of the laminations in eachlayer. This is about as far as lap joints can be closed without takingvery special care in making the joint and with out applying excessivepressure to close it.

In Fig. 11, the curves 35, 36, and 31 show the relation between excitingcurrent and joint opening in inches for the same three cores havingrespectively the standard reversible pattern two way overlap and thethree and four-way irreversible overlap shown in Figs. 2 and 4. In curve35 the exciting current goes up very rapidly and practically doubles foreach twentieth of an inch of opening. Curve 36 for the three-way lapjoint shows that exciting current is no higher with an opening ofone-quarter inch (.250 than is the exciting current for the standardcore when it is open only .050 inch. Curve 3'! for the four-way overlapshows a still further reduction in sensitivity of excitin current tojoint opening. All the curves in Fig. 11 were taken at a flux density of100 kilolines per square inch.

In Fig. 12, curves 35', 36', and 31' correspond to curves 35, 3B, and31, except that the flux density is 80 kilolines per square inch. Hereit will be seen that there is a marked difference between the standardtwo-way lap joint on the one hand and the three and four-way lap jointson the other hand. There is practically no difference between the lattertwo and, further- I more, the exciting current is practically the samefor the latter two at an opening of .25 inch as it is when the joint ispractically fully closed.

It has also been found that cores which are provided with this noveljoint are substantially quieter than similar cores which are providedwith the same number of conventional joints such as two-way lap joints.The results of noise tests are shown in Fig. 13, in which curve 38 showsthe relationship between core noise in decibels and core flux densityfor a given size core having two standard two-way lap joints. Curve 39shows the decibel flux-density characteristic of the same size corewhich is provided with a four-way lap joint of the type shown in Fig. 4and it will be seen that the noise which is produced at correspondingflux densities is substantially lower in the case of this joint. Curve40 is for the same size core which has the joints in the individuallamination layers distributed at random throughout the entire core sothat, in effect, the core has no joint. It will be seen from acomparison of curves 39 and 40 that the four-way lap joint is almost asquiet as no joint at all.

While there have been shown and described particular embodiments of theinvention, it will be obvious to those skilled in the art that changesand modifications can be made without departing from the invention and,therefore, it is aimed in the appended claims to cover all such changesand modifications as fall within the true spirit and scope of theinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a laminated magnetic core, a closed loop comprising at least sixradially nested fiatwise curved lamination layers of magnetic stripmaterial, each of said layers having at least two closely adjacent endsof said strip material which between them form a high specificreluctance section, said high reluctance sections collectivelycomprising a lap joint extending entirely across the core perpendicularto the plane of the laminations and extending along the length of thecore parallel with the plane of laminations for a dis tance which is asmall fraction of the total length of the magnetic circuit of the core,said lap joint being characterized by a three step pattern which isirreversibly repeated, the corresponding steps in each three steppattern beginning in the same plane perpendicular to the plane oflaminations.

2. In a laminated magnetic core, a closed loop comprising at leasttwelve radially nested flatwise curved lamination layers of magneticstrip material, each of said layers having at least two closely adjacentends of said strip material which between them form a gap, said gaps insuccessive layers being offset in a pattern which irreversibly repeatsat least every fourth layer, the length of the offsets being greaterthan the separation of the gaps, the repetitive pattern forming at leastthree groups of longitudinally separated gaps, which groups lierespectively in diflerent planes which are perpendicular to the surfaceof the layers.

3. The core recited in claim 1 in which there are at least twosuperposed laminations in each layer.

4. The core recited in claim 2 in which the lengthwise separations ofthe two outermost of said planes is short in comparison with the radialthickness of said core.

5. In a magnetic core having a plurality of lamination layers, a jointwhose length is short in comparison with the thickness of said core,said joint comprising a separated butt joint between the ends oflamination pieces in each lamination layer, said butt joints beingstaggered in an asymmetrically recurring pattern which includes at leastthree successive lamination layers so that in a perpendicular planethrough any group of transversely aligned separated butt joints at leasttwo-thirds of the cross sectional area of the core' will be magneticmaterial.

6. In combination, a closed magnetic core including a plurality ofradially nested, fiatwise curved layers of magnetic material, aseparable and closable joint in said core, said joint including a gap ineach layer, all of said gaps being parallel with each other said gaps insuccessive layers being offset lengthwise by greater distances than thegap lengths, the offsetting being in a regular pattern whichirreversibly repeats at least every fourth layer and at most every sixthlayer; said patter-n being characterized by arranging all of said gapsinto at least three substantially equal numbered groups of gaps, thegaps in each group being aligned in a plane substantially perpendicularto the surface of the layers, the transverse separation between adjacentgaps in each group being equal, the lengthwise separation of said planesbeing substantially uniform whereby at least two-thirds of the corecross sectional area in each of said planes is magnetic material.

7. The combination recited in claim 5 in which there are at least twosuperposed laminations in each layer.

GARETH G. SOMERVILLE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,154,866 Lunnon et al. Sept. 28,1915 2,290,680 Franz July 21, 1942 2,305,650 Vienneau Dec. 22, 19422,456,458 Somerville Dec. 14, 1948 FOREIGN PATENTS Number Country Date106,986 Great Britain June 14, 1917

